The present invention relates generally to airway adapters used with gas analyzers for monitoring constituent respiratory gases in a patient. The present invention relates particularly to an airway adapter with a surface confronting the airway which interacts with water in the patient's breath so as to reduce the impact of water on the monitoring of the constituent gases.
It has long been recognized in the anesthetic and other health care related arts that monitoring respiration is a very reliable method of determining the condition of a patient and, moreover, is one which provides an instant indication of adverse conditions as compared with other life signs which take longer periods to depart from normal values. Such an immediate indication is of great interest in connection with patients undergoing surgery or who are in other life-threatening situations, and in connection with monitoring infants who are subject to cessation of breathing (apnea) for no apparent cause.
Respiratory gas analyzers function by passing electromagnetic radiation (typically light) of a specific wavelength (typically infrared) through the respiratory gas and measuring the absorption for a component being monitored (such as CO.sub.2). One method of monitoring breathing which has been used extensively involves monitoring the difference in carbon dioxide (CO.sub.2) content between an individual's inspired and expired gas streams. It is impossible to breathe without the carbon dioxide content of the expired gas stream varying from that of the inspired gas stream by at least about 2%.
Capnography, the measurement of carbon dioxide levels in the airway, is one particular monitoring technology that aids clinicians in critical patient management decisions. Capnography assists clinicians in patient management decisions by providing the means to assess a large number of problems related to ventilation, cardiopulmonary functions and metabolism. Capnography can help clinicians monitor the integrity of gas delivery systems and mechanical ventilators as well. It can provide early warning of events which may indicate an obstruction of the patient's airway tube or disconnection of the ventilatory circuit, the onset of pulmonary disease or changes in physiologic status. In addition, waveforms displayed on a monitor corresponding to carbon dioxide concentrations may be used by clinicians to assess emphysema, asthma, chronic bronchitis, pneumonia and cardiac arrest.
Most capnometers are comprised of an airway adapter (sometimes referred to as a cuvette), an emitter, a detector, and a processor. The constituent gas flows through the airway adapter and between the emitter and detector, which are placed behind windows on either side of the airway adapter. Measurement is made directly through the windows. The detector receives the energy that is transmitted by the emitter that is not absorbed by the constituent gas, and the processor processes the signal from the detector. For example, emitted infrared light of a selected wavelength band is attenuated in correspondence to the quantity of carbon dioxide in the respiratory gas.
U.S. Pat. No. 4,648,396 (Raemer) discloses a respiration detector which features an infrared source and detector pair disposed on opposite sides of a cuvette through which the gas stream is inhaled and exhaled by a patient. U.S. Pat. No. 5,067,492 (Yelderman et al.) discloses a disposable endotracheal airway adapter that connects in series with a respirator or anesthesia breathing circuit and allows the passage of infrared radiation through the gases for measuring the constituent concentration of respiratory gases.
Conventional capnometers typically have problems associated with moisture buildup, calibration and warm-up time, which problems tend to limit their widespread acceptance. Respiratory gas contains substantial amounts of water vapor which condenses on the inner sides of the airway adapter and on the windows through which the light is passed for measuring a gas component. The gas analysis is impeded by this presence of saturated water vapor. In the closed environment of a patient ventilator, water continuously condenses on all surfaces of the airway adapter including the windows. Liquid water present on the airway adapter windows strongly absorbs infrared light at the wavelengths used to measure carbon dioxide, anesthetic agents and other gases, therefore making the measurement of carbon dioxide and other agents less precise and in some cases unreliable. The presence of individual liquid water droplets on the windows further contributes to such problems due to reflection and diffraction of the light used for measurement.
A conventional method of preventing condensation on airway adapter windows has been to heat the windows. U.S. Pat. No. 5,092,342 (Hattendorff et al.) discloses a sensor arrangement including a housing containing a transmitter, a receiver, and a heatable holder for a measuring cuvette. U.S. Pat. No. 5,146,092 (Apperson et al.) discloses an airway adapter with a heater provided to heat the casing of the airway adapter associated with a transducer to a temperature high enough to keep condensation from forming on the windows.
Heating the cuvette has a number of disadvantages. First, heating requires significant amounts of power, which is particularly limited in battery operated systems. Second, additional power and sensing lines increase the cost and weight of the system. Third, the mechanical design becomes complex as heat transfer issues are encountered. Lastly, warm-up time can delay the use of the sensor in emergency applications.
There is a continuing and long-felt need to provide a cost-effective way to prevent the condensation of water vapor on the inside of the windows in an airway adapter from interfering with the accuracy and reliability of measuring component respiratory gases.